An X-ray detector that is a planar radiation detector using an active matrix or a solid-state imaging element such as CCD, CMOS, etc., is drawing attention as a new-generation X-ray diagnostic image detector. By irradiating X-rays onto the X-ray detector, an X-ray image or a real time X-ray image is output as a digital signal.
The X-ray detector includes a photoelectric conversion substrate that converts light into an electrical signal, and a scintillator layer that contacts the photoelectric conversion substrate and converts the X-rays incident from the outside into light. Then, the light of the incident X-rays converted by the scintillator layer reaches the photoelectric conversion substrate and is converted into charge; and the charge is read as an output signal and converted into a digital image signal by a prescribed signal processing circuit, etc.
In the case where the scintillator layer includes CsI which is a halide, simple CsI cannot convert the incident X-rays into visible light; therefore, similarly to a general fluorescer, an activator is included to activate the excitation of the light due to the incident X-rays.
In the X-ray detector, because the peak wavelength of the light reception sensitivity of the photoelectric conversion substrate exists at the vicinity of 400 nm to 700 nm in the visible light region, in the case where CsI is used in the scintillator layer, Tl is used as the activator because the wavelength of the light excited by the incident X-rays in Tl is at the vicinity of 550 nm.
In the case where the scintillator layer is a fluorescer of CsI containing Tl as an activator and the CsI is a halide, similarly to a fluorescer containing a general activator, the characteristics of the scintillator layer are greatly affected by the concentration and concentration distribution of Tl which is the activator.
In an X-ray detector or a scintillator panel including the scintillator layer containing the activator, in the case where the concentration and concentration distribution of the activator are not corrected, this causes characteristic degradation of the scintillator layer, affects the afterimage (the phenomenon in which the subject image of the X-ray image of the (n−1)th time or earlier remains in the X-ray image of the nth time), and affects the sensitivity (the luminous efficiency) relating to the light emission characteristics of the scintillator layer.
For example, because the imaging conditions are greatly different between the subjects in the diagnosis using the X-ray image (the ray amount of the incident X-rays being about 0.0087 mGy to 0.87 mGy (because the X-ray transmittance is different between sections)), a large difference may occur in the ray amount of the incident X-rays between the X-ray image of the (n−1)th time and the X-ray image of the nth time. Here, in the case where the ray amount difference of the incident X-rays of the X-ray images of the (n−1)th time and the nth time is (n−1)>n, the afterimage occurs because the light emission characteristics of the scintillator layer of the non-subject portion of the X-ray image of the (n−1)th time change due to the large energy of the incident X-rays; and the effects remain through the X-ray image of the nth time.
For the diagnosis using the X-ray image, the afterimage characteristics are important characteristics even when compared to other characteristics of the scintillator layer such as the sensitivity (the luminous efficiency) and the resolution (the MTF).
In the diagnosis using the X-ray image, normally, there are many cases where the diagnosis is performed in the state in which the subject is disposed at the central portion of the X-ray image; therefore, the characteristics in the central region of the formation region of the scintillator layer are important.
Conventionally, there have been proposals to regulate the concentration and concentration distribution of the activator of the scintillator layer to improve the sensitivity (the luminous efficiency) and the resolution (the MTF).